1. Field of the Invention
The present invention relates to a radio burst signal transmission system.
2. Description of the Related Art
FIG. 1 of the accompanying drawings shows a general arrangement of a radio burst signal transmission system. In FIG. 1, terminal 20 belongs to base station 10. For upstream communications, terminal 20 sends individual information burst signal 30 to base station 10. If base station 10 properly receives individual information burst signal 30, then base station sends ACK (Acknowledgment) signal 40 to terminal 20. If terminal 20 receives ACK signal 40, then terminal 20 sends new individual information burst signal 30 to base station 10, if necessary. If terminal 10 does not receive ACK signal 40, then terminal 20 resends individual information burst signal 30 to base station 10.
For downstream communications, a similar exchange of signals takes place between base station 10 and terminal 20. Specifically, base station 10 sends individual information burst signal 31 to terminal 20. If terminal 20 properly receives individual information burst signal 31, then terminal 20 sends ACK signal 41 to base station 10. If base station 10 receives ACK signal 41, then base station 10 sends new individual information burst signal 31 to terminal 20, if necessary. If base station 10 does not receive ACK signal 41, then base station 10 resends individual information burst signal 31 to terminal 20.
FIG. 2 of the accompanying drawings shows an example of the format of a radio burst signal used in the radio burst signal transmission system. The radio burst signal transmission system performs synchronization and propagation path estimation using a known signal added to the leading end of the burst signal shown in FIG. 2. The radio burst signal transmission system also determines information required to demodulate the burst signal, such as the length of the burst signal and the modulation scheme thereof, using a burst information signal of the bust signal. The radio burst signal transmission system demodulates the burst signal based on the synchronization and the propagation path estimation that have been performed and the information that has been determined.
A process of controlling the radio burst signal transmission system for high transmission efficiency will be described below. Generally, a radio propagation path between a transmitter and a receiver in the radio burst signal transmission system is represented by a function of a frequency component f in a frequency band that is used and a time dependent variation t of the frequency component f. That is, the radio propagation path is represented by a function F(f,t). For demodulating a burst signal that has been propagated through the radio propagation path F(f,t), the radio propagation path is estimated using a known signal added to the leading end of the burst signal. If the radio propagation path is estimated at a time t=0, then the radio propagation path estimated using the known signal is represented by F(f,t=0). A data signal contained in the burst signal is demodulated using F(f,t=0).
With the above demodulation process, since the radio propagation path F(f,t) is expressed as the function of the time t, the difference ΔF(t) between F(f,t) and F(f,t=0) tends to increase with the time. This may be attributed to a phase shift due to a fluctuation of the oscillation frequency of the oscillator and a disturbance such as an amplitude/phase shift due to fading. The increase of the difference ΔF(t) also increases the amplitude and phase errors of the demodulated signal, making the probability of a burst demodulation error greater toward the trailing end of the burst signal.
Prior art solutions to the abovementioned problem include performing an adaptive transmission rate control process based on the probability of successful reception of a successful reception indication signal, referred to as ACK, indicating the successful reception of a radio packet signal. According to the adaptive transmission rate control process, the number of bits to be assigned per symbol is set to an optimum value depending on the probability of successful reception of ACK. Specifically, if the probability of successful reception of ACK is higher, then the number of bits to be assigned per symbol is increased to increase a transmission rate. On the other hand, if the probability of successful reception of ACK is lower, then the number of bits to be assigned per symbol is reduced in order to suppress a reduction in the transmission rate due to a packet signal error, thereby increasing the robustness against disturbances and reducing an error rate of the burst signal.
There are available different types of the adaptive transmission rate control process in the art. They include, for example, an adaptive modulation control process to select an optimum modulation signal from BPSK (Binary Phase Shift Keying), QPSK (Quadrature Phase Shift Keying), 16 QAM (Quadrature Amplitude Modulation), and 64 QAM signals depending on the quality of the received signal, and an adaptive encoding ratio control process to set an encoding ratio to an optimum value depending on the quality of the received signal.
With the adaptive transmission rate control process based on the probability of successful reception of ACK, if the probability of successful reception of ACK is higher, then the quality of the received signal is judged as high, and the number of bits to be assigned per symbol is increased or the encoding ratio is increased for communications thereby to increase the transmission efficiency. On the other hand, if the probability of successful reception of ACK is lower, then the quality of the received signal is judged as low, and the number of bits to be assigned per symbol is reduced or the encoding ratio is reduced for communications thereby to reduce the frequency of resending requests due to a burst signal error to increase the transmission efficiency.
The adaptive transmission rate control process is usually performed on a fixed number of data per burst signal. When the probability of successful reception of ACK is lowered and the transmission rate is reduced, the number of pits transmitted per unit time is reduced, resulting in an increase in the length of the burst signal.
In general, when the length of a burst signal increases, the burst signal reception rate tends to increase because the difference ΔF(t) increases by increase (a). If a reduction (b) in the burst signal reception rate due to an increase in the distance between minimum signal points resulting from a reduction in the transmission rate or an increase in the encoding ratio is greater than the increase (a), then the burst signal reception rate is reduced as a whole, increasing the transmission efficiency. Conversely, if the reduction (b) is smaller than the increase (a), then the transmission efficiency is reduced.
In order to improve the reduction in the transmission efficiency, it has heretofore been proposed to perform a fragmentation control process, which is a type of the adaptive transmission rate control process, for divisionally transmitting burst signals. For details, see JP-A-2002-44135, for example. According to the fragmentation control process, the length of each burst signal can be shortened for divisionally transmitting burst signals. As a result, an increase in the error ΔF(t) which tends to increase with time can be suppressed. As a consequence, the burst signal reception error rate is lowered, and the frequency of resending requests for a burst signal is lowered, thus increasing the transmission efficiency.
In summary, the conventional control processes for increasing the transmission efficiency include an adaptive transmission rate control process (hereinafter referred to as “first control process”) and a fragmentation control process (hereinafter referred to as “second control process”). The first control process is more suitable for use as a control process as the error ΔF(t) becomes more time-uncorrelated. The second control process is more suitable for use as a control process as the error ΔF(t) becomes more time-correlated.
The conventional adaptive transmission rate control process depending on the probability of successful reception of ACK is performed solely based on information indicative of whether or not the received burst signal is in error. Therefore, the conventional adaptive transmission rate control process is problematic in that it is unable to determine with accuracy whether the optimum control process for improving the transmission efficiency is the first control process or the second control process. If the first control process is to be selected as an optimum control process, but the second control process is actually selected in error, then the burst signal reception error rate is not reduced by shortening the length of each burst signal, and the transmission efficiency is not increased. Conversely, if the second control process is to be selected as an optimum control process, but the first control process is actually selected in error, then the transmission rate is lowered to increase the length of each burst signal, so that the difference ΔF(t) increases with time, the burst signal reception error rate increases, and the transmission efficiency is not increased.